The invention relates to a differential pressure sensor made using glass-silicon technology with a high overload resistance for industrial applications.
For measuring differential pressures, usually piezoresistive or capacitive pressure sensors are used. A common characteristic of both is that a diaphragm is deformed pressure-dependently. The degree of deformation is in this case a measure of the pressure.
Piezoresistive pressure sensors are distinguished by high long-term stability, a wide operating temperature range and a large measuring range in conjunction with low temperature dependence and high measurement dynamics. However, particularly in the case of high pressures or differential pressures, piezoresistive pressure sensors have an unsatisfactory resistance to overloading.
DE 200 19 067 discloses a pressure-measuring device with a piezoresistive pressure sensor and hydraulic force transmission in which the process pressure of the measuring medium is transmitted to the pressure sensor by interposing a separating diaphragm with a fluid diaphragm seal, the process-pressure-dependent, diaphragm-seal-displacing deflection of the separating diaphragm being mechanically limited to an amount prescribably exceeding the measuring range, and the pressure sensor being arranged in the pressure-measuring device in such a way that it can move on a mechanically pretensioned overload diaphragm which, in dependence on process pressure exceeding the measuring range, limits a volumetrically variable equalizing space for accepting excess diaphragm seal.
This construction is complex and also characterized by a large number of joining processes between components subjected to pressure, which place extreme demands on the joint, in particular in the case of high limit pressures. Industrial applications of differential pressure sensors require overload resistance up to 400 bar.
DE 42 07 949 discloses a capacitive differential pressure sensor made using glass-silicon technology in which a plate of silicon, serving as a pressure-sensitive diaphragm and as a first electrode, is arranged between two carrier plates consisting of glass, the plate and the carrier plate being integrally connected to one another in their edge region by anodic bonding in such a way that in each case a carrier plate combines with the plate serving as the diaphragm to form a measuring chamber, each carrier plate has a pressure supply duct, which runs perpendicular to the contact surfaces of the plate and of the carrier plates and via which the respective measuring chamber can be pressurized, and the surfaces of the carrier plates lying opposite the deflectable region of the plate serving as the diaphragm are each provided with a metallization, serving as a second electrode, in such a way that the first electrode and the second electrodes form a differential-pressure-dependent capacitor arrangement.
The differential-pressure-dependent deformation of the plate serving as a diaphragm brings about a change in capacitance of the capacitor arrangement, the change in capacitance being a direct measure of the differential pressure. The change in capacitance is measured electrically. To allow a wide measuring range to be covered with adequate measuring accuracy, it is necessary for the deflectable region of the plate serving as a diaphragm to have a displacement which is at odds with designing the differential pressure sensor to be resistant to overloading. Industrial applications of differential pressure sensors demand overload resistance up to 400 bar.
In contrast thereto the differential pressure sensor of the present invention has high overload resistance in conjunction with high resolution at the beginning of the measuring range.
The invention proceeds from a known capacitive differential pressure sensor made using glass-silicon technology, in which a diaphragm plate of silicon, serving as a first electrode and with a pressure-sensitively deflectable region, is arranged between two carrier plates consisting of glass, the diaphragm plate and each carrier plate being integrally connected to one another in their edge region by anodic bonding in such a way that in each case a carrier plate combines with the diaphragm plate to form a measuring chamber, each carrier plate has a pressure supply duct, which runs perpendicular to the contact surfaces of the diaphragm plate and of the carrier plates and via which the respective measuring chamber can be pressurized, and the surfaces of the carrier plates lying opposite the deflectable region of the diaphragm plate are each provided with a metallization, serving as a second electrode, in such a way that the first electrode and the second electrodes form a differential-pressure-dependent capacitor arrangement.
The essence of the invention consists in that the diaphragm plate has for a prescribed measuring range within the same measuring chambers a plurality of mutually independent deflectable regions as measuring diaphragms for in each case a part-sensor with a part-measuring range, the overlapping of all the part-measuring ranges of the part-sensors being equal to the prescribed measuring range of the differential pressure sensor, the displacement of the measuring diaphragm of each part-sensor being mechanically limited outside its part-measuring range.
The measuring range of the differential pressure sensor is made up of the part-measuring ranges of the individual part-sensors. In this case, the high resolution in the part-measuring range of each part-sensor contributes to the resolution of the differential pressure sensor over the entire measuring range. In a corresponding way, the resolution at the beginning of the measuring range of the differential pressure sensor is determined by the resolution of the part-sensor with the part-measuring range for lowest differential pressures. The number of part-sensors is governed by the width of the measuring range of the differential pressure sensor and required resolution over the measuring range. With an increasing number of part-sensors, the measuring range of the differential pressure sensor is increased while the resolution remains the same and, within a prescribed measuring range of the differential pressure sensor, the resolution is increased.
Consequently and advantageously, a single differential pressure sensor is sufficient for a large number of different industrial applications. As a result, the expenditure in production and stockkeeping is reduced as a result of a smaller number of different individual parts and higher unit numbers of the single differential pressure sensor, this also being the case in service.
If the applied differential pressure exceeds the measuring range of a part-sensor by a prescribable amount, the measuring diaphragm of this part-sensor comes to bear against the nearest carrier plate. Consequently, the measuring diaphragm of this part-sensor is effectively protected from being damaged by overload.
According to a further feature of the invention, the part-measuring ranges are formed by part-sensors following one another in the measuring range and overlapping one another at the measuring range limits. In the measuring range limiting band produced as a result, the differential pressure is measured by two part-sensors of neighboring part-measuring ranges. It is obvious here that the two part-sensors must produce the same measured value for differential pressures in the measuring range limiting band of successive part-measuring ranges.
This partial redundancy advantageously achieves the effect of confirming measured values of the part-sensors for differential pressures in the measuring range limiting bands of successive part-measuring ranges.
According to a further feature of the invention, the various part-measuring ranges of the part-sensors are set by the rigidity of the measuring diaphragm, adapted to the respective part-measuring range. The dependence of the respective part-measuring range on the rigidity of the measuring diaphragm achieves the same maximum displacement for all the part-sensors of the differential pressure sensor.
Consequently, for overload protection, the mechanical displacement limitation for all the part-sensors of the differential pressure sensor is advantageously situated identically in one plane.
According to a refining feature of the invention, the rigidity of the measuring diaphragm is set by the diaphragm surface area. In this case, the diaphragm thickness of the measuring diaphragm is the same for all the part-sensors. With the same diaphragm thickness, measuring diaphragms with a smaller diaphragm surface area have a greater rigidity than measuring diaphragms with a larger diaphragm surface area. The measuring diaphragms of the part-sensors with part-measuring ranges designed for high differential pressures have a greater rigidity than the measuring diaphragms of the part-sensors with part-measuring ranges designed for low differential pressures.
In this case, all the measuring diaphragms are advantageously able to be formed during production by a single depth structuring process. With a diaphragm plate of silicon, it is advantageous to bring about the depth structuring by etching. In this case, the etching depth is proportional to the etching duration. With the same diaphragm thickness of the measuring diaphragm for all the part-sensors, all the measuring diaphragms are structured in a single etching process of the same duration for all the measuring diaphragms.
According to an alternative refining feature of the invention, the diaphragm thickness and the surface area of the measuring diaphragm are the same for all the part-sensors and each measuring diaphragm has reinforcing structures, in dependence on the respective part-measuring range.
This advantageously succeeds in accommodating a large number of part-sensors on a diaphragm plate of small surface area. This feature is particularly advantageous in the case of differential pressure sensors for a wide measuring range in conjunction with high resolution over the entire measuring range.